This Nonprovisional application claims priority under 35 U.S.C. § 119(a) on Patent Application No(s). 2003-030639 filed in JAPAN on Feb. 7, 2003, the entire contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a II–VI compound semiconductor single crystal, and particularly to a CdTe-base compound semiconductor single crystal useful as a material of an electro-optic element used for a voltage detector or the like.
2. Description of Related Art
In an earlier development, it is known that a CdTe-base compound semiconductor single crystal belonging to II–VI compound semiconductor is useful for a radiation detector (for example, refer to Japanese Patent Application Publication (Examined) No. Tokukouhei H07-96478). When the CdTe-base compound semiconductor single crystal is used as a base material of the radiation detector, the crystal used for the detector is desired to have high resistivity (for example, 1.0×109 Ω·cm or above) for the purpose of achieving high resolution for high energy while reducing noises.
Generally, to heighten resistivity of the CdTe-base compound semiconductor single crystal, a method of doping halogen such as chlorine or the like into the crystal is used. The inventors of this specification propose CdTe single crystal which is obtained by doping a predetermined amount of chlorine into the CdTe single crystal to heighten resistivity of the CdTe single crystal to 1.0×109 Ω·cm or above (refer to Japanese Patent Application No. Tokugan 2002-076313).
Further, it is known that the CdTe-base compound semiconductor single crystal is an electro-optic material having a property of changing refractive index in proportional to a voltage applied to the single crystal (for example, refer to Journal of Nonlinear Optical Physics and Materials Vol. 5, No. 2 (1996) pp. 247–268). The electro-optic material having the property is, for example, used as an electro-optic element such as a voltage (or electric field) detector or the like (for example, refer to Japanese Patent Application Publication (Examined) No. Tokukouhei H06-95108, Japanese Patent Application Publication No. Tokkaihei H05-267408, Japanese Patent No. 2947288 and Japanese Patent Application Publication (Examined) No. Tokukouhei H07-96478).
Hereinafter, an example of the voltage detector will be described with reference to FIG. 5. FIG. 5 is a view schematically showing the configuration of a general voltage detector.
A voltage detector 100 shown in FIG. 5 has an optical probe (electro-optic element) 10 closely placed in adjacent to or making in contact with a measured subject, an optical section 30 for guiding a light beam outgoing from a light source 31 to the optical probe 10, and a comparison and detection section 20 for comparing a reference beam output from the light source 31 and an outgoing beam transmitted through and returned from the optical probe 10.
The optical probe 10 has an electro-optic material 11 made of a compound semiconductor single crystal, a connection unit 12 for guiding the beam transmitted from the optical section 30 into the electro-optic material 11, a metal thin film 13 attached to the top of the electro-optic material 11, and a conductive electrode 14 arranged on the outer circumference of the electro-optic material 11. The comparison and detection section 20 has an analyzer 21, photoelectric conversion elements 22 and 23, a comparison circuit 24 and an output terminal 25. The optical section 30 has the light source 31, a polarizer 32, beam splitters 33 and 34, a collimator 35 and an optical fiber 36.
When the optical probe 10 is closely placed in adjacent to or makes in contact with the measured subject, an electric field is induced between the conductive electrode 14 set to an appropriate electric potential (for example, ground potential) and the metal thin film 11, and a refractive index of the electro-optic material 11 is changed by the electric field. Thereafter, the polarization state of the beam transmitted through the electro-optic material 11 is changed in dependent on the change of the refractive index of the electro-optic material 11.
In the voltage detector 100 described above, when a light beam is output from the light source 31 such as a laser diode or the like driven by direct current, only a light beam L having a predetermined polarization component is extracted from the output light beam by the polarizer 32, and the light beam L is divided into a reference beam L1 and an incident beam L2 by the beam splitter 33.
The incident beam L2 is guided to the optical probe 10 through the beam splitter 34, the collimator 35 and the optical fiber 36 and is incident on the electro-optic material 11 through the connecter unit 12. Thereafter, the incident beam L2 is transmitted through the electro-optic material 11 and is reflected on the metal thin film 13. Thereafter, the reflected beam is transmitted backward through the electro-optic material 11, the connecter unit 12, the optical fiber 36 and the collimator 35 and is incident on the beam splitter 34 as an outgoing beam L3. Thereafter, the outgoing beam L3 is reflected on the beam splitter 34 and is incident on the analyzer 21. Thereafter, the outgoing beam L3 is converted into an electric signal in the photoelectric converter element 22. The reference beam L2 obtained in the beam splitter 33 is incident on the photoelectric converter element 23 and is converted into an electric signal.
Thereafter, the electric signals output from the photoelectric converter elements 22 and 23 are compared with each other in the comparison circuit 24, and a intensity ratio signal indicating a ratio of the level of the electric signal of the photoelectric converter element 22 and the level of the electric signal of the photoelectric converter element 23 is output from the output terminal 25 and is obtained. That is, the polarization state of the beam transmitted through the electro-optic material 11 is changed in dependent on the change of the refractive index of the electro-optic material 11, and the intensity of the outgoing beam L3 is changed with the polarization state of the beam. Therefore, a voltage of a predetermined portion of the measured subject can be detected by comparing the intensity of the outgoing beam L3 and the intensity of the reference beam L1.
In the Applications No. H6-95108, No. H7-96478 and No. H5-267408 and the Patent No. 2947288, the compound semiconductor single crystals such as lithium tantalate (LiTaO3), lithium niobate (LiNbO3), KTP (KTiOPO4), arsenic gallium (GaAs) and the like are used.
However, though CdTe-base compound semiconductor used as the electro-optic material is known, CdTe-base compound semiconductor is not generally used as a material of the electro-optic element such as a voltage detector. That is, to use CdTe-base compound semiconductor for the electro-optic element, though CdTe-base compound semiconductor having a property of high resistivity and high transmittance (low absorption coefficient) is needed, CdTe-base compound semiconductor having the property is not known.